Question

The following are two methods on an object I’ve created. The superclass has implemented MouseListener.

@Override public void mousePressed(MouseEvent event){
    mIsPushed = true;
    System.out.println("Button pushed");
  }

  @Override public void mouseReleased(MouseEvent event){
    mIsPushed = false;
    System.out.println("Button released");
  }

When the two System.out.println calls are uncommented, the program works fine and behaves as it should. When they aren’t, it does nothing. I have even placed breakpoints at the mIsPushed assignments and they’re never reached.

However, if I put a breakpoint at the point where these two functions are called, the debugger reaches it and the calls are made.

Can anyone tell me what I’ve missed?

Answer 1

This is probably due to the different threads caching their own values of the mIsPushed variable.

You need to synchronize the access to this variable. You have a few different options:

• Declare it as volatile
• Use some high level construct from java.util.concurrent package (such as AtomicBoolean)
• Synchronize the access to the variable using the synchronized keyword.

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In more detail: The Java memory model does not guarantee that a read of a variable from one thread will yield the last value written to that variable from another thread. This is to allow a JVM to optimize the execution of a Java program. Adding a System.out.println may flush the cache and cause a new value to be loaded when needed.

To explicitly tell the JVM to make sure that no cached value should be used, you need to introduce a “happens before”-relation between the write and the read.

Further reading:

• Java Language Specification: Threads and Locks